# Momentum and Collisions - Mission MC4 Detailed Help The law of conservation of momentum applies to any collision between two objects that occurs in an isolated system. Assuming that the collision between Object A and Object B occurs in an isolated system, which of the following statements are consistent with this law? Momentum is said to be conserved for collisions occurring in an isolated system. But what exactly does this mean? If you're not sure of the meaning of momentum conservation, then you're likely to miss this question. Read on!   During a collision, an object encounters an impulse that changes its momentum. Each object changes its momentum. So certainly it is not a single object that conserves its momentum. It is the system of two colliding objects that conserves momentum. When considered together, the total amount of momentum of both colliding objects is conserved. If the momentum of the two objects are added together (as vectors with their + and - signs), then the total system momentum can be determined. After the collision, there will be the same amount of total system momentum. One object may lose momentum; but the other object gains the momentum that was lost. Thus, momentum never leaves the system; it is simply transferred from one object to the other. (See Know the Law section.) The Law of Momentum Conservation: If a collision occurs between object 1 and object 2 in an isolated system (see definition in the Define Help section), then the momentum change of object 1 is equal in magnitude and opposite in direction to the momentum change of object 2. In equation form   m1 • ∆v1 = - m2 • ∆v2 The total momentum of the system before the collision (p1 + p2) is the same as the total momentum of the system of two objects after the collision (p1' + p2'). That is   p1 + p2 = p1' + p2' Total system momentum is said to be conserved for any collision occurring in an isolated system. Isolated System and External Forces: A system of two colliding objects is considered to be an isolated system if the only momentum-changing forces exerted during the collision are the forces between the two objects themselves. If a third object exerts a force capable of changing one or both of the object's momentum during the collision, then this is considered an external force and the system is not isolated.  